Evolution and recovery of original total organic carbon for muddy source rocks with different total organic carbon in the Western Sichuan Depression, Sichuan Basin, China

According to the new quantitative model for original total organic carbon (TOCo) evolution and recovery of source rocks, The TOCo evolution of muddy source rocks with different TOCs (MSRDTOC) (0.5% < TOC ≤ 1.5%, 2.5% < TOC ≤ 3.5% and 4.0% < TOC ≤ 6.0%, and their average values were 1.0%, 3.0% and 5.0%, respectively) and type III kerogen were investigated in the Western Sichuan Depression (WSD) of Sichuan Basin, China, finally recovery coefficients of TOCo were calculated. Studies showed that TOCo evolution of the MSRDTOC have similarities and differences. The similarities reveal that with the increase of thermal evolution level, varied TOC all reduce gradually and the sharp reduction occurs at the stage of a large number of hydrocarbon generation and expulsion (0.7% ≤ vitrinite reflectance (VR) ≤ 1.3%). The differences indicate that the starting time and decrease range for varied TOC are different. For TOCo = 1.0%, 3.0% and 5.0%, their starting time of TOC reduction are VR = 1.05%, 0.80% and 0.60%, respectively. When VR reaches to 3.0%, their TOC decrease ranges are 30.54%, 38.52% and 44.00% and their recovery coefficients are 1.44, 1.63 and 1.79, respectively. The higher the TOC is, the earlier the starting time of TOC reduction is, the bigger the range of TOC reduction and recovery coefficient are. It must restore TOCo when we evaluate source rocks at high-over mature stage and predict oil and gas resources. The MSRDTOC and their TOCo recovery should be evaluated, respectively.